Washington (CNN) -- The air traffic controller suspended for failing to respond to two planes heading into Ronald Reagan Washington National Airport has told investigators that he had fallen asleep, according to the National Transportation Safety Board.

The controller, a 20-year veteran, "indicated that he had fallen asleep for a period of time while on duty," according to a statement released Thursday by the safety board. "He had been working his fourth consecutive overnight shift (10:00 p.m. to 6:00 a.m.)."

"Human fatigue issues are one of the areas being investigated," the statement read.

Federal Aviation Administration chief Randy Babbitt said earlier Thursday that the FAA and the National Transportation Safety Board are investigating the incident and that the air traffic controller has been suspended from all operational duties.

An FAA official speaking on background said the controller was given a drug test after the incident. The official said the drug test was "standard procedure" and did not know the results.

NTSB: Controller fell asleep

How safe are air traffic control towers?

The situation began at 12:10 a.m. Wednesday, when an American Airlines plane attempted to call the tower to get clearance to land and got no answer, said Peter Knudson, a spokesman for the safety board. The plane had been in contact with a regional air traffic control facility, and a controller at that facility advised the pilot that he, too, had been unable to contact anyone at the tower, according to a recording of air control traffic at the website liveatc.net.

"1012," the controller said, using the airline's flight number, "called a couple of times on landline and tried to call on the commercial line, and there's no answer.

"The tower is apparently unmanned."

Apparently asked why by a pilot, the controller later responded, "Well, I'm going to take a guess and say that the controller got locked out. I've heard of this happening before. Fortunately, it's not very often," he said.

Knudson said the plane landed without incident in a situation termed an "uncontrolled airport."

About 15 minutes later, a United Airlines flight also failed to reach the tower but landed without any problems, he said. After that, the controller in the tower was back in communication. Knudson said one controller was staffing the tower at the time this occurred.

The controller's admission that he was asleep during the landing emergency underscores concerns about the effect of fatigue on underslept controllers at work.

In 2007, then-NTSB Chairman Mark Rosenker wrote in a letter to the FAA that four plane incidents "provide clear and compelling evidence" that controllers are sometimes operating while fatigued because of their work schedules and poor use of rest periods.

"That fatigue has contributed to controller errors," Rosenker wrote.

The incidents cited by the NTSB were:

-- A March 23, 2006, incident in which a Chicago air traffic controller cleared a plane to take off from a runway on which, 15 seconds earlier, he had cleared another aircraft to cross. The pilot of the departing plane stopped when he saw the other craft in the taxiway intersection. The controller told investigators he had slept only four hours during a nine-hour break between shifts.

-- An August 19, 2004, incident in which a Los Angeles controller cleared one passenger jet to take off and another to land on a runway at the same time. The pilot in the landing aircraft noticed the other on the runway and pulled his plane up 12 seconds before they would have collided. The controller said he had slept five or 6 hours before coming to work.

-- A September 25, 2001, incident in which a Denver air traffic controller approved a request from a cargo plane pilot to take off from a runway that had been closed for construction. The aircraft came within 32 feet of hitting lights that had been installed in the construction zone. The controller said he'd slept only two hours between work days.

-- A July 8, 2001, incident in which a Denver controller cleared one passenger plane to cross a runway where another was about to land. The landing pilot hit the brakes, stopping 810 feet from the other plane. The controller said he had worked three shifts in two days.

Of the most recent incident, Babbitt said, "In my 25 years as a professional airline pilot, I've never seen anything happen like this.

"I am outraged by it," Babbitt said. "We're going to make sure something like this never happens again."

Babbitt stressed that, because of a backup system, neither plane was out of "positive radar contact, nor were they out of communication with the FAA, thus allowing both to land safely.

"That said ... this should not have happened," Babbitt said. "We should not have had this gap in communication. We had to rely on a backup system, which shouldn't have happened."

Transportation Secretary Ray LaHood ordered the FAA on Wednesday to schedule two controllers on the overnight shift.

"It is not acceptable to have just one controller in the tower managing air traffic in this critical air space. I have also asked ... Babbitt to study staffing levels at other airports around the country," he said.

Knudson said it's not uncommon for planes to land at uncontrolled airports. He said control towers at some fields across the country shut down for the night, and planes still land. However, he could not comment on whether that practice was ever used at Reagan National.

The American Airlines flight, which was coming from Miami, had 91 passengers and six crew members aboard, airline spokesman Ed Martelle said. The United Airlines flight was arriving from Chicago with 63 passengers and five crew members, spokeswoman Megan McCarthy said.

American Airlines had no comment on the situation, saying it was leaving it to the FAA to handle. United Airlines noted that the National Transportation Safety Board is reviewing the incident, and McCarthy said the airline is conducting its own review.